In a power amplifier for a car audio system having a battery as a power source, a reference potential of an input signal is biased with direct-current and an input signal source and a differential input circuit are separated in a direct-current manner by an input coupling capacitor.
Meanwhile, battery voltage for a car is to be in an instantaneous-drop voltage state as instantaneously dropping from a steady state of 12 V due to much flow of load current for starting operation of various instruments at the time of engine starting, for example. At that time, fluctuation of the direct-current bias occurs at the power amplifier driven by the battery due to charge/discharge electric current transiently flowing through the input coupling capacitor. The fluctuation of the direct-current bias generates raspy pop noise.
With a power amplifier in the related art, in order not to generate such pop noise, fluctuation of the direct-current bias has been compensated by connecting an AC coupling capacitor which is charged and discharged at the same timing with an input coupling capacitor between a reference side input of a differential input circuit and a grand potential.
Here, when the battery voltage is in an instantaneous-drop voltage state and voltage required for normal operation of the differential input circuit (i.e., headroom voltage) cannot be ensured, output operation of the power amplifier is to be unstable. Accordingly, a power amplifier in the related art is to be in a non-operable state when the battery voltage is in the instantaneous-drop voltage state. Therefore, there has been a problem of occurrence of an audio dropout.
Particularly, since a system of idling stop (i.e., of engine stop) when a car is stopped is becoming popular due to a recent trend of ecology, the audio dropout has been occurring frequently due to frequent occurrence of an instantaneous drop of the battery voltage.